Offset printing processes consist of indirect methods wherein an ink is transferred from a printing plate to a blanket cylinder and then said ink is transferred onto a substrate. Accordingly, the blanket cylinder is inked by the printing plate. Offset printing takes advantage of the difference in surface energy between the image area and the non-image area of the printing plate. The image area is oleophilic, whereas the non-image area is hydrophilic. Thus, oily inks used in the method tend to adhere to the image-area and to be repelled from the non-image area of the printing plate. Wet offset printing is typically carried out by feeding both a fountain solution (also referred in the art as dampening solution) and an oleophilic ink to the printing plate surface to allow the image areas to receive preferentially the ink and the non-image areas preferentially the fountain solution and then transferring the ink deposited on image areas onto a substrate.
In a conventional wet offset printing process, the printing plate is damped with a fountain solution thus increasing the difference in surface energy between the image and the non-image area of the printing plate, thereby enhancing the ink repellency of the non-image area and the ink receptivity of the image. In such a process, water forms a film on the hydrophilic areas (i.e. the non-image areas) of the printing plate but contracts into tiny droplets on the water-repellent areas (i.e. the image areas). When an inked roller is passed over the damped printing plate, it is unable to ink the areas covered by the water film but it pushes aside the droplets on the water-repellant areas and these ink up. In other words, fountain solutions are used to separate the image and non-image areas so as to prevent the transfer of ink onto non-image areas of the printing plate. Moreover, the fountain solution has to fulfill various tasks including the wetting of the non-image area quickly, uniformly and without excess; quickly producing a homogeneous emulsion with the oily ink; protecting the printing plate against corrosion and wear and maintaining a low and constant temperature in the ink train.
Oxidative drying inks (i.e. inks which dry by oxidation in the presence of oxygen, in particular in the presence of the oxygen of the atmosphere) are typically used during offset printing processes. Oxidative drying inks preferably comprise catalysts or driers (also referred in the art as siccatives, siccative agents, desiccatives or dessicators) to set up the oxidation process. Examples of driers include inorganic or organic salts of metal(s), metallic soaps of organic acids, metal complexes and metal complex salts. Known driers comprise metals such e.g. cobalt, copper, manganese, cerium, zirconium, barium, strontium, lithium, bismuth, calcium, vanadium, zinc, iron and mixtures thereof. In particular, cobalt salts are widely used as driers for inks and coatings due to their high oxidative drying efficiency and their robustness, i.e. their efficiency remains independent of the coating compositions.
Failure of the ink to dry rapidly results in set off. Set off occurs when printing ink which is not dry adheres to the back of a printed substrate placed on top of it during the stacking of printed substrates as it comes off the presses (see e.g. U.S. Pat. No. 4,604,952). This is a particular problem in the use of off-set printing processing for printing security features. Bank notes and other security documents typically carry a multitude of overlapping security features which are applied one after the other. If the previously applied security feature, e.g. a background image or graphic pattern, has not yet sufficiently dried, the whole multi-step printing process is delayed.
Catalysts comprising other metals, such as e.g. manganese, cerium, zirconium, bismuth, calcium, zinc and iron, have been used as catalysts for the drying process of oxidative drying inks. However, their oxidative drying efficiency tends to be weaker as compared to cobalt catalysts. Moreover, these catalysts' robustness is more restricted as compared to the conventional cobalt catalysts.
There is some increasing concern about cobalt containing driers for reasons of health and environment issues. With the implementation of REACh, (eco)-toxicological studies are being conducted in Europe. In this frame, cobalt compounds have been increasingly scrutinied in particular with regards to their potential effects on the environment and on the reproduction. For instance, the environmental toxicity of the widely used drier cobalt ethylhexanoate has been raised in 2010 from being classified as “Toxic to the environment” (Hazard statements H401: toxic to aquatic life/H413: may cause long-lasting harmful effects to aquatic life) to “Very toxic to the environment” (Hazard statements H400: very toxic to aquatic life). As a consequence, products containing as low as 0.25% of cobalt ethylhexanoate must already be reclassified themselves as “Toxic to the environment” (information note on cobalt ethylhexanoate, CEPE, December 2010). Furthermore, it is as well classified as H361f (suspected of damaging fertility), which would trigger the classification of any mixture containing it at more than 0.3% as reprotoxic.
In an attempt to provide driers that are more friendly to health and environment, a variety of compounds have been developed. Catalysts comprising other metals, such as e.g. manganese, cerium, zirconium, bismuth, calcium, zinc and iron, have been used as catalysts for the drying process of oxidative drying inks. However, their oxidative drying efficiency tends to be weaker as compared to cobalt catalysts. Moreover, the robustness of these catalysts is more restricted as compared to the conventional cobalt catalysts.
Manganese containing compounds have been developed as driers for coatings or inks. E. Bouwman and R. van Gorkum disclose complexes of manganese, pentadione and bipyridyl as driers for alkyd paints, in particular for the oxidative crosslinking of ethyl linoleate (J. Coat Technol Res 4(4) (2007, 491-503). WO 2008/003652 A1 and WO 2011/083309 A1 disclose catalysts based on iron-manganese complexes containing polydentate ligands for air-drying alkyd-based resins. EP 1 564 271 B1 discloses driers consisting of a combination of iron and manganese salts of fatty acids. WO 2011/098583 A1, WO 2011/098584 A1 and WO 2011/098587 A1 disclose oxidative drying coating compositions comprising polymers containing unsaturated fatty acid residues and manganese salts complexes as drying catalysts.
Recent developments in the field of driers for oxidizing alkyds useful as polymeric binders have been reviewed by Soucek and Wu in Progress in Organic Coatings (2012) 73, 435-454. However, none of these driers is as reactive and universal as the cobalt containing driers known in the art. Alternative driers also frequently tend to produce undesired yellowing of the dried coating. Moreover, alternative driers often cause storage stability problem related to skin formation inside the ink container and require the addition of increased concentrations of anti-skinning agents.
WO 2014/086556 A1 discloses oxidative drying inks suitable for offset, letterpress and intaglio printing, wherein said oxidative drying inks comprise at least one oxidative drying varnish and one or more neutral manganese complex compounds. The disclosed inks are said to combine short drying times while exhibiting good non-yellowing characteristics upon use and time and while being environmentally friendly.
With the same aim of accelerating the drying process, it has been the practice to add driers, in particular cobalt-containing driers, to the fountain solution. Even though the ink and the fountain solution are immiscible, a certain amount of fountain solution is invariably transferred from the plate to the inking rollers. The driers are carried thereby into the inking system and become emulsified in the ink (see e.g. U.S. Pat. No. 3,354,824).
As mentioned hereabove, wet offset printing processes use fountain solutions. In the light of concerns about cobalt-containing compounds for reasons of health and environment (for example, cobalt acetate is classified as SVHC) and since high amounts of fountain solutions are used and consequently high amounts of waste are produced, there is a strong need for environmentally friendly fountain solutions.
JP 2001341458 A discloses fountain solutions for lithographic printing processes, said fountain solutions comprising a fatty acid metal salt as drier to accelerate the drying process of oxidative drying inks on paper. Since the disclosed fatty acid metal salts are not water soluble, they are absorbed on porous grains, in particular hydrophobic silica particles having an average size between 0.1 μm and 10 μm, which are dispersed in water. Accordingly, such hydrophobic particles are prone to precipitation thus leading to fountain solutions suffering from a lack of stability upon storage and use on the offset printing machine. The description mentions that for example cobalt, manganese, lead, iron, calcium, cerium or rare earth metals can be used as metal component of the fatty acid metal salt. The sole examples disclosed in JP 2001341458 A are cobalt-containing fatty acid salts.
US 2004/211333 discloses that inorganic salts of peracids may be used in inks and fountain solutions. In particular, coatings made of these inks are said to be fast drying, having a reduced or no VOC components, and have reduced or no toxic metal-containing components.
Thus, there remains a need for environmentally friendly inks and fountain solutions for wet offset printing processes, wherein both inks and fountain solutions combine stability upon storage at room temperature and stability at low temperature upon use on the printing machine without impacting the drying performance of the applied ink to produce security features on a substrate.